The technical field of this invention is dielectrometry and, in particular, the spatial interrogation of materials to deduce their physical properties from complex permittivity measurements.
Dielectric measurement of materials during manufacturing and the like is becoming increasingly commonplace. For example, on-line monitoring devices are now available for measuring the progress of curing in parts molded from polymeric composites. Such monitors typically are embedded into a portion of the part which will be trimmed off during subsequent finishing operations. By measuring changes in the complex dielectric permittivity of the composite, the state of curing can be deduced. Such sensors can be used to control or modify various manufacturing parameters, such as the heating and/or cooling rates, or the amount of applied pressure.
It is known in the art that A.C. measurements of dielectric properties by sensors implanted within a curing polymer can provide useful data on curing and other material properties. In particular, U S. Pat. No. 4,423,371 issued to Senturia et al. in December, 1983, discloses that A.C. measurements in the frequency range of 1 Hz to 10 kHz can be reliable indicators of curing. See also U.S. Pat. No. 4,399,100 issued to Zsolnay et al. in August, 1983, and U.S. Pat. No. 4,496,697 issued to Zsolnay et al. in January, 1985, for further disclosures of automatic process control systems for curing polymeric materials.
Conventional sensors for measuring changes in the dielectric properties of a curing polymer are typically either formed as simple parallel plate capacitors, such as those disclosed in U.S. Pat. Nos. 4,399,100 or 4,496,697 or planar interdigitated capacitors, such as those disclosed in U.S. Pat. No. 4,423,371. The utility of these sensors is limited by their inability to provide enough information to resolve distributions of parameters. The deduction of inhomogeneities is sometimes attempted from temporal frequency response information, but this process does not give a unique distribution and cannot be implemented without assuming that the frequency dispersion of the material in question is known.
There exists a need for better permittivity measuring devices and methods. In particular, there exists a need for better devices and methods for conducting non-destructive interrogations of materials to determine their physical properties across a spatial profile.